This invention relates to the preparation of ordered organic molecular layers both on flexible and rigid substrates. In particular the invention relates to methods for the preparation of thin light polarization films of lyotropic liquid crystal, which can be used, for example, as the internal polarizers, as well as dichroic polarizers, for the production of liquid crystal displays (LCDs).
Light-polarization films or polarizers are major components of liquid crystal displays (LCDs) and other liquid crystal (LC) devices. Common polarizers are based on polyvinyl-alcohol-iodine (PVA) films of 30-50 xcexcm thickness. These polarizers are generally placed on the external glass surfaces of the LC cell and require protective films (e.g. cellulose triacetate or cellulose acetate butyrate). Their fabrication is rather complicated and expensive. The external placement of the polarizers results in additional reflections and parallax effect, which deteriorates the LCD contrast, optical performance and viewing angles. Consequently, thin internal polarizers for LCDs are highly desirable. However, this variant still cannot be realized on the basis of conventional PVA films.
FR 2,186,165 discloses a method of forming internal polarizers by coating a long linear polymer film (e.g. PVA) to the internal surfaces of the glass substrates. The polymeric solution is then subjected to a linearly mechanical deformation (e.g. using a rubber rod), giving rise to a preferential direction parallel to the substrate plane. This results in an ordering of the long polymeric molecules along the direction of the deformation. Subsequently, the ordered molecular state can be fixed after the evaporation of the solvent. The final polarization film is obtained by a volume impregnation of iodine vapor or iodine solution or a deposition of a dichroic dye. This method is complicated, unreliable and inefficient for LCD manufacturing processes. One of the disadvantages of the method is the diffusion of the iodine molecules into the bulk of the LC. Thus, it leads to the deteriorated resistivity, increased power consumption and diminished life time of the LCD.
Lyotropic liquid crystals (LLCs) can also be used for the purpose of the preparation of thin polarization films. The LLCs in an organic solvent can be coated on the glass substrate by a mechanical shear flow. After the evaporation of the solvent, the molecular order is maintained in LLC solid film.
Another method is described in U.S. Pat. No. 2,524,286 and 5,739,296 (FIG. 1). The isotropic solution of the lyotropic dye 5 is deposited from the tank 3 onto the anisotropic surface I of the flexible polymer film 2. So a thin film 4 of the dye solution can be formed. Low-cost polyethyleneterephthalate (PET) film can be used as the polymer film 2. The other variants include the deposition onto the thin layer (0.1-0.5 xcexcm) of paraffin wax, mineral oil, barium stearate, a resin or other materials. Afterwards, the dye layer is oriented by rubbing or brushing to form the anisotropic film 4. The organic solvent can be composed of water dissolved with low molecular weight solvents, such as acetone, alcohol, dioxane etc. First the ordered nematic LLC phase is formed in the film 7 after the partial evaporation of the solvent 8. The final evaporation of the solvent 9, when baking the film 7, results in a highly ordered solid film of the lyotropic dye 11 with good extinction ratio. The typical thickness of LLC polarizers is 0.3-0.5 xcexcm, which is comparable with the thickness of usual LC alignment layers. The thickness can be regulated by the gap 14 between the die 15 and the surface 1 of the substrate 2.
The rate of the evaporation is an important factor. Rapid evaporation at a high temperature results in a xe2x80x9cboilingxe2x80x9d of the solution, while a slow evaporation rate at a low temperature leads to the formation of randomly oriented dye polycrystals, thus affecting the optical quality of the light polarization films.
An adhesive layer is used for the transfer of the polarization film from the flexible polymer carrier to the glass substrate of an LCD cell. The adhesive may be pressure sensitive or other permanently tacky type which is rendered tacky by the application of water or other solvent, by heat or other means. For the tacky type adhesives, chlorinated latex, poly-isobutylene of low molecular weight etc. may be used.
Light polarization films of a large size can be formed, as can leaves of a medium size and special forms, which can be cut from the plane leaves. A volume form is also possible, if convex or concave substrates are applied e.g. for lenses, lamp bulbs etc.
This technology makes it possible to fabricate multi-layer light-polarization structures with specific orientations of the optical axes in each layer as well as polychromatic absorption properties. The polarization film possesses a high radiation stability, high temperature stability (up to 200xc2x0 C.), high color fastness and UV-stability. These properties make such films attractive for the replacement of iodine based external polarizers in current LCD production.
During the manufacturing process, however, the appearance of defects in the form of horizontal stripes several to ten microns wide is possible. The defects divide the areas with different molecular orientations which are clearly seen in a polarized light as the vertical bands. The sources of such defects are the turbulence of LLC flow, non-uniform properties of the alignment layer and non-optimal deposition conditions. Minimizing the defects is possible by changing the deposition speed and LLCs viscosity, using the corona discharge to prepare the substrate, reducing the shift rate of the substrate and fixing the deposition device position.
USSR Pat. No 697,950 proposes the formation of the internal polarizers using LLCs. The procedure describes the deposition of the lyotropic gel of 1-30 wt/wt % on the surface of the transparent electrodes. The gel is then subjected to a shear flow in the velocity of 102-107 secxe2x88x921, e.g. by spin coating. The procedure is followed by baking-out the solvent.
The proposed technology allows the use of thin light polarization film based on LLCs for the internal LCD polarizers. However the polarization direction cannot be made arbitrarily to follow a specific local distribution, e.g. to follow a mosaic picture with the characteristic size of tens of microns or less. This limitation is due to the poor spatial resolution of the proposed method.
The purpose of the proposed invention is to develop a new technology for the fabrication of thin neutral or color polarizers, with a desired distribution of the polarization axis, for LCD applications.
The objective of the present invention is to provide a new technology for the fabrication of thin photo-patterned (pixelated) polarizers with a desired local distribution of the polarization axis.
According to the present invention there is provided a method of forming a thin light polarization film on a substrate, comprising the steps of: (a) depositing a thin solid film polarizer onto a flexible polymeric carrier sheet, (b) applying a photo-curable glue onto said substrate, (c) bringing said thin solid film polarizer into contact with said glue, (d) illuminating and curing said glue, and (a) removing said carrier sheet.
In a preferred embodiment the step of illuminating and curing said glue is carried out by illuminating said glue in a pattern whereby a pattern of cured glue is formed and whereby when said carrier sheet is removed said thin solid film polarizer only remains attached to said glue in said pattern. Regions of glue that are not cured may be removed by a solvent.,
Preferably the illumination is carried out through a patterned mask. This mask may be a shadow mask or a photomask formed by photolithography.
In a preferred embodiment of the invention the light polarization film is formed on the substrate in a pattern formed by regions of at least two different directions of polarization. This may be achieved by: (a) depositing a first solid thin film polarizer on a first flexible carrier, said first polarizer having a first polarization direction, (b) applying a photo-curable glue to said substrate, (c) bringing said first solid thin film polarizer into contact with said glue, (d) illuminating said glue in a first pattern to form a pattern of cured glue having said first solid thin film polarizer adhered thereto, (e) removing said first flexible carrier leaving said first solid thin film polarizer adhered to said substrate by said glue in said first pattern, (f) depositing a second solid thin film polarizer on a second flexible carrier, said second polarizer having a second polarization direction, (g) applying a photo-curable glue to said substrate, (h) bringing said second solid thin film polarizer into contact with said glue, (i) illuminating said glue in a second pattern to form a pattern of cured glue having said second solid thin film polarizer adhered thereto, and (j) removing said second flexible carrier leaving said second solid thin film polarizer adhered to said substrate by said glue in said second pattern.
In preferred forms of the invention the light polarization film is divided into pixels having different light polarization directions, and these pixels may be divided into sub-pixels, each sub-pixel being formed with different absorption colors. Alternatively, all the pixels may have the same polarization direction.
The flexible carrier sheet may be formed of an isotropic or non-isotropic polymeric material, and may include a detachment layer. Preferably the detachment layer also serves as a polarization alignment layer. The detachment layer may comprise a film of material selected from the group consisting of paraffin wax, mineral oils, barium stearate, resins, uniaxially aligned polyethyleneterephthalate or the like. This detachment and alignment layer can be rubbed mechanically to obtain a desired orientation.
The thin light polarization film is preferably formed on a substrate forming the inner surface of a liquid crystal display.
According to the present invention there is further provided a method of forming a thin light polarization film comprising the steps of: (a) depositing a layer of photoalignable material on a substrate, (b) illuminating the photoalignable layer with actinic radiation to define a polarization axis of said photoalignable layer, (c) applying a thin layer of an isotropic absorber solution onto said photoalignable layer, (d) partially evaporating said solution to form a gel, and (e) baking said gel to form an anisotropic absorber layer.
In one embodiment of the invention the actinic radiation is linearly polarized and the principal absorption axis of said photoalignable layer is orthogonal to the polarization vector of said actinic radiation. In another embodiment of the invention the actinic radiation is non-polarized and is incident on said photoalignable layer at an oblique angle.
Preferably the photoalignable layer is illuminated through a mask whereby only selected regions of said layer are aligned. More preferably still, the photoalignable layer is illuminated through several masks in sequence whereby different regions of said photoalignable layer may be formed with different alignment axes. The photoalignable layer may be illuminated through a photo-patterned mask that transforms linearly polarized or non-polarized actinic radiation into actinic radiation having a spatial distribution of polarization vectors, and this photo-patterned mask may be a light polarization mask or a birefringence mask.
Preferably more than one absorber material may be provided and different absorbers may be chosen with different colors. The absorber may comprise lyotropic liquid crystal. The photoalignable material may preferably be an organic azodye.
The photoalignable material may be deposited in a layer of from 0.05 to 1.5 xcexcm thick, while the absorber material may have a thickness of from 0.3 to 1.5 xcexcm.
Preferably the thin light polarization film is formed on a substrate forming an inner surface of a liquid crystal cell.
Viewed from a still further aspect the present invention provides a thin light polarization film deposited on a substrate and comprising a plurality of pixels, wherein said pixels are formed with different axes of polarization. This light polarization)film may be formed on the internal surface of a substrate defining a liquid crystal cell.